In the design of electrical circuits, there is often a need to provide electrical reactance in the circuit. Such reactance is usually provided by way of a magnetic device, such as an inductor, comprised of one or more windings of an electrical conductor, (i.e., a wire or strip of metal). When the inductor must carry high currents, as is common in power supply circuits, the resistance of the inductor should be minimized, typically by increasing the cross-sectional area of the conductor which forms the windings. Minimizing the inductor resistance is even more important when the constraint of reduced size is imposed.
The cross-sectional area of each winding can be maximized by constructing the inductor of a flat metallic strip wound in a spiral. The problem associated with constructing an inductor in this fashion is bringing the inner end of the inductor outside the spiral in order to make an electrical connection therewith, while minimizing loss of the conductor cross-sectional area. One possible solution to this problem is disclosed in U.S. Pat. No. 4,959,630, issued in the names of A. J. Yerman et al., which describes a spiral coil formed of a metallic conductor laminated to a pliable dielectric material. The metallic conductor is patterned in a continuous chain of undulating, end-to-end semicircles.
It is believed that there are several disadvantages in the approach proposed by Yerman et al. First, the inductor of Yerman et al. is not wound, but rather, is formed by folding each semicircle over another in an accordion-like fashion so that a small amount of winding volume is lost in each fold. Moreover, the conductor of the Yerman et al. inductor patent is believed to be constrained to thickness on the order of about three mils (76.2.mu.). For high-frequency operation, such a conductor thickness is probably sufficient because the conductivity is limited by the skin-depth effect. However, at lower frequency operation, a greater conductor thickness is probably necessary.
Another possible solution to the problem of how to bring the inner spiral end out from the coil is to attach a terminal to both ends of the coil. The addition of such a terminal adds to the fabrication cost of the device and causes a decrease in the conductivity of the windings at the junction with the terminal. Such a conductivity decrease is attributable to the fact that solder is less conductive than the copper typically used to form the windings.
There is a need for a spiral coil which avoids the disadvantages of the prior art.